1330 H. Tomaszewski piezospectroscopic technique+(spectrometer Dilor biaxial, tensile stresses were expected in the layer X4800) with the greater thermal expansion(zirconia). Pie zospectroscopic measurements showed that the distribution of compressive stresses was different in 3 Results and discussion both types of barrier layers. In the alumina layers compressive stress was not only a function of the Controlled crack growth experiments indicated layer thickness but also of the position across the significant crack deflection in barrier layers of layer(Figs 3 and 4). Maximum of compressive composites dependent on barrier composition but stress was observed at the interface(280 MPa)and not on type of zirconia matrix. It occurred that minimum in the center of alumina layer. The crack deflects only in alumina layers(Figs I and 2). minimum stress was dependent on alumina layer In zirconia layers crack deflects back to its original thickness (Table 1). On the contrary, in ba arrie direction perpendicular to the layers. The degree of crack deflection depends on alumina layer thick ness(Table 1). As it was shown at Figs I and 2 in barrier layers made of an oxide mixture crack pro- agates perpendicularly to the layers without 28 deflection independently on layer thickness Generally crack deflection is related to residual thermal stresses and instantaneous elastic modulus 表属 mismatch effects. In the present case the residual biaxial compressive stress was expected in the layer with lower thermal expansion coefficient(alumina), 普1636 Fig. 2. Crack path during fracture of Ce-TZ/Al2O3 composite with 50 um thick barrier layers made of alumina(bottom, Table 1. Crack deflection angle and compressive stress in minimum in alumina layers of Y-TZP/Al,O3 composite as a function of layer thickness 85l Thickness of Crack deflection Compressive stress alumina laver 22±5 229·2 Fig. 1. Crack path in 60 um thick alumina layer(bottom, 40 62±8 1219 inverted image)and in 45 um thick barrier layers made of an 60 91-6 oxide mixture(top, normal age)of Y-TZP/Al2O3 compositepiezospectroscopic technique4 (spectrometer Dilor X4800). 3 Results and Discussion Controlled crack growth experiments indicated signi®cant crack de¯ection in barrier layers of composites dependent on barrier composition but not on type of zirconia matrix. It occurred that crack de¯ects only in alumina layers (Figs 1 and 2). In zirconia layers crack de¯ects back to its original direction perpendicular to the layers. The degree of crack de¯ection depends on alumina layer thick￾ness (Table 1). As it was shown at Figs 1 and 2 in barrier layers made of an oxide mixture crack pro￾pagates perpendicularly to the layers without de¯ection independently on layer thickness. Generally crack de¯ection is related to residual thermal stresses and instantaneous elastic modulus mismatch e€ects. In the present case the residual biaxial compressive stress was expected in the layer with lower thermal expansion coecient (alumina), biaxial, tensile stresses were expected in the layer with the greater thermal expansion (zirconia). Pie￾zospectroscopic measurements showed that the distribution of compressive stresses was di€erent in both types of barrier layers. In the alumina layers compressive stress was not only a function of the layer thickness but also of the position across the layer (Figs 3 and 4). Maximum of compressive stress was observed at the interface (280MPa) and minimum in the center of alumina layer. The minimum stress was dependent on alumina layer thickness (Table 1). On the contrary, in barrier Fig. 1. Crack path in 60m thick alumina layer (bottom, inverted image) and in 45 m thick barrier layers made of an oxide mixture (top, normal age) of Y-TZP/Al2O3 composite. Fig. 2. Crack path during fracture of Ce-TZ/Al2O3 composite with 50m thick barrier layers made of alumina (bottom, inverted image) and an oxide mixture (top, normal image). Table 1. Crack de¯ection angle and compressive stress in minimum in alumina layers of Y-TZP/Al2O3 composite as a function of layer thickness Thickness of alumina layer (m) Crack de¯ection angle () Compressive stress in minimum (MPa) 10 0 266.8 25 22‹5 229.2 40 62‹8 121.9 60 90 91.6 1330 H. Tomaszewski